1. Field of the Invention
This invention relates to a device to guarantee the sequence of movement of at least two fluid-actuated displacement units, with a primary displacement unit moved chronologically before a secondary displacement unit, and with the primary displacement unit in the form of a cylinder having a retractable and extendable piston rod.
2. Technical Considerations
A device of the known art is described in DE 101 25 351 A1 and is provided in that case for use in lifting masts of industrial trucks (e.g., fork-lift trucks).
In lifting masts of this type, a primary lifting cylinder in the form of a plunger piston cylinder that is initially extended is connected with load holding means, such as a fork carrier. A secondary lifting cylinder (or two secondary lifting cylinders) that is also in the form of a plunger piston cylinder that is extended after the primary lifting cylinder is engaged on a mast that can be moved vertically and is guided in a stationary mast. The sequence in which the piston rods of the primary and secondary lifting cylinders are extended is determined by the piston surface area, with the primary lifting cylinder being extended first on account of its larger piston surface. Only when the piston rod of the primary lifting cylinder has been fully extended or comes in contact against a limit stop does the piston rod of the secondary lifting cylinder begin to be extended as a result of the increase in the system pressure.
On account of the different piston surface areas of the lifting cylinders, the lifting speeds are different, which can be avoided by the use of lifting cylinders that have equal areas. In this case, the sequence of extensions is determined exclusively by the dead weight of the components to be lifted by the lifting cylinders.
Regardless of the relationship between the piston surface areas, it can occur under unfavorable conditions, in particular at low oil temperatures, that the piston rod of the secondary lifting cylinder is extended first. This reversal of the desired sequence of movement can also occur when the lifting frame is retracted. Although, in that case, the situation is considered less critical.
In one exemplary embodiment described in DE 101 25 351 A1, two secondary lifting cylinders are connected in parallel to a pressure medium source and a primary lifting cylinder is connected in series downstream of one of the secondary lifting cylinders. Upstream of the other secondary lifting cylinder is a stop valve that opens as a function of the pressure. As a result of which, the pressure difference between the primary lifting cylinder and the second secondary lifting cylinder is increased and a reversal of the desired sequence of movement is prevented.
Therefore, it is an object of this invention to provide a device of the general type described above but that has a simplified construction.